Foveapeltis gen. nov., an unusual cleroid genus with large hypomeral cavities from mid‐Cretaceous amber (Coleoptera: Cleroidea)

Abstract Beetles have a remote evolutionary history dating back to the Carboniferous, with Mesozoic fossils playing a pivotal role in elucidating the early evolution of extant families. Despite their exceptional preservation in amber, deciphering the systematic positions of Mesozoic trogossitid‐like beetles remains challenging. Here, we describe and illustrate a new trogossitid‐like lineage from mid‐Cretaceous Kachin amber, Foveapeltis rutai Li, Kolibáč, Liu & Cai, gen. et sp. nov. Foveapeltis stands out within the Cleroidea due to the presence of a significant large cavity on each hypomeron. While the exact phylogenetic placement of Foveapeltis remains uncertain, we offer a discussion on its potential affinity based on our constrained phylogenetic analyses.


| INTRODUC TI ON
Cleroidea is a moderately diverse superfamily of cucujiform beetles, with 18 families as recognized by Gimmel et al. (2019).
Several Mesozoic genera preserved as adpression fossils have at least once been associated with the trogossitid-like groups (Kolibáč, 2013;Schmied et al., 2009Schmied et al., , 2011)).However, their familial attribution is often difficult to evaluate due to the poor state of preservation, especially considering that the trogossitid-like families redefined by Gimmel et al. (2019) often lack clear and easy-to-observe apomorphies (e.g., Kolibáč, 2013;Yu et al., 2014Yu et al., , 2015)).Recently discovered amber fossils preserve greater details facilitating classification; nevertheless, the mixture of diagnostic characters of different families still makes the familial assignment thorny in some cases.Cretamerus Peris et al. from mid-Cretaceous French amber was suggested to be possibly related to Decamerini (Thymalidae), although this hypothesis was not unequivocally demonstrated by their cladistic analysis (Peris et al., 2014) (Kolibáč & Peris, 2021;Lyubarsky et al., 2021;Yu, Leschen, et al., 2021;Yu, Li, et al., 2021).In the case of Gracilenticrus where a cladistic analysis was done, however, the assignment to Lophocateridae was not proved (Yu, Li, et al., 2021).
Zaiwa also exhibits features unknown among the extant lophocaterids, making its placement suspicious.Microtrogossita Li & Cai from Kachin amber was assigned to Trogossitidae as demonstrated by a cladistic analysis, although it still possesses some unusual features such as the relatively widely separated pro-and mesocoxae (Li et al., 2021).
Here, we describe another unusual group of cleroid fossils from mid-Cretaceous Kachin amber, which in body shape resembles some members of the trogossitid-like families, but bears characters hitherto unknown in the whole Cleroidea.

| Materials
The Kachin amber (Burmese amber) specimens studied herein (Figures 1-9) originated from amber mines near Noije Bum Academy of Sciences (IZGAS), Guangzhou, China.When necessary, the amber pieces were trimmed with a small table saw, ground with emery papers of different grit sizes and finally polished with polishing powder.

| Fossil imaging
Brightfield images were taken with a Zeiss Discovery V20 stereo microscope.Confocal images were obtained with a Zeiss LSM710 confocal laser scanning microscope, using the 488 nm Argon laser excitation line (Fu et al., 2021).Images were stacked with Helicon Focus 7.0.2,Zerene Stacker 1.04 and Adobe Photoshop CC, and were further processed in Adobe Photoshop CC to adjust brightness and contrast.Microtomographic data for the specimen NIGP203568 were obtained with a Zeiss Xradia 520 Versa 3D X-ray microscope at the micro-CT laboratory of NIGP and analyzed in VGStudio MAX 3.0.Scanning parameters were as follows: isotropic voxel size, 1.7815 μm; power, 3 W; acceleration voltage, 40 kV; exposure time, 4 s; projections, 3001.

| Description and measurement
The morphological terminology generally follows Lawrence and Ślipiński (2013) and Kolibáč (2005Kolibáč ( , 2013)).It should be noted that the position of the mandibular apical teeth in the horizontal or vertical axis always refers to the configuration in dorsal/ventral view in Kolibáč (2005Kolibáč ( , 2006)), while in some other coleopteran publications (e.g., Escalona et al., 2020;Hörnschemeyer, 2009) it refers to the configuration in apical view.Microtrogossita actually has mandibular teeth horizontally situated in dorsal/ ventral view, and was incorrectly coded in the analysis by Li et al. (2021).
The length and width of body parts are affected by the viewing angle and the conformation of the beetle body.Therefore, here only the body length (BL) and body width (BW) are provided as rough indicators of the overall body size.The measurements of body length were taken as the apparent distance from mandibular apex to elytral apex in dorsal view.The measurements of body width were taken as the pronotal width, even though in the specimen NIGP203568 the width across the detached elytra appears to be superficially larger.

| Phylogenetic analyses
As shown by previous studies (e.g., Gimmel et al., 2019;Kolibáč, 2006;Li et al., 2021), the phylogenetic relationships among the trogossitidlike groups cannot be properly resolved based on morphological information alone (i.e., unconstrained morphology-based analyses).Thus, to evaluate the systematic placement of the new fossil genus, we conducted constrained morphology-based phylogenetic analyses under maximum parsimony.The use of molecular-based constraints would allow a more realistic estimation of the states at ancestral nodes, and therefore contribute to a more authentic placement of the fossil (Fikáček et al., 2020).The data matrix for extant genera was taken from Li et al. (2021), which was derived from Kolibáč (2006Kolibáč ( , 2008)).The full matrix includes 61 adult and 32 larval characters, among which we successfully coded 31 adult characters for the new fossil (File S1).The constraining backbone tree was created based on the Bayesian molecular tree by Li et al. (2021), who re-analyzed Gimmel et al.'s (2019) data with the site-heterogeneous model CAT-GTR + G4.Site-heterogeneous models could generally improve the accuracy of phylogenetic analyses (e.g., Li, Engel, et al., 2023), and specifically, compared with Gimmel et al. (2019), Li, Kolibáč, Liu & Cai, gen. et sp. nov., holotype, NIGP203568, under 2021) is still not fully compatible with the phylogenomic studies.However, the results of these phylogenomic studies were not directly used here due to the much sparser taxon sampling.
The parsimony analyses were performed under both equal and implied weights, using R 4.1.0(R Core Team, 2021) and the R package TreeSearch 1.3.1 (Smith, 2023).The concavity constant in the weighted analyses was set to 12, following the suggestion by Goloboff et al. (2018) and Smith (2019).
In the first analysis (File S2), all taxa in the morphological matrix were included.For taxa with both morphological and molecular data, their interrelationships were fixed as the backbone tree.
In the second set of analyses (File S3), only the taxa present in the backbone tree and the fossil genus were included.Only the fossil genus was allowed to move freely across the backbone tree.In order to perceive the uncertainty of the fossil placement, the parsimony scores of the trees with alternative placements of the fossil were mapped to the corresponding branches of the backbone tree (Li et al., 2024).The results were visualized with the R package ggtree 6.5.2 (Yu, 2020;Yu et al., 2017) and graphically edited with Adobe Illustrator CC 2017.Etymology.The generic name is formed based on the Latin "fovea," pit, referring to the large hypomeral cavities, and Peltis Müller, a well-known cleroid genus, referring to the overall similar habitus.The name is feminine in gender.

Remarks.
In the right elytron of specimen NIGP203568, the two inner carinae merge together near the elytral base.We regard this as an individual variation, as its left elytron is the same as other specimens, with all carinae isolated.The specimen IZGAS-BA-COL003 appears to have a more distinctly punctured prosternum compared to NIGP203568 and IZGAS-BA-COL001.However, in extant beetles, the punctation could vary within one species.Thus, here we decide to not establish a separate species based on IZGAS-BA-COL003, especially considering that no other good differential characters have been found for it.

| RE SULTS
In the best tree under implied weights (Figure 10), Foveapeltis is resolved as the sister group of Phloiophilus (Phloiophilidae).As the best tree is not substantially better than the alternatives, it would be informative to also explore the suboptimal options.With the parsimony score of other possibilities labeled on the backbone tree, it appears that Foveapeltis may also be related to Thymalus (Thymalidae) or Eronyxa (Lophocateridae), or located in the basalmost part of the sampled taxa (Figure 11).
The remaining cleroid groups are more or less similar to Foveapeltis in habitus.The monogeneric Peltidae shares with Foveapeltis the longitudinal elytral carinae, but differs from the latter in the larger body size, the presence of frontoclypeal suture, complete elytral epipleura, and protibiae with a large curved spur at apex and spines along outer edge (Kolibáč, 2013).Thymalidae as defined by Gimmel et al. (2019) (Crowson, 1964); Phloiophilidae can be differentiated from Foveapeltis in the procoxae slightly projecting (Kolibáč, 2008); Thymalinae can be differentiated from Foveapeltis in the body strongly convex, head mostly covered by pronotum and elytral epipleura wide and complete (Asakawa et al., 2020;Asakawa & Yoshitomi, 2019).Besides, the elytra of Thymalidae, Protopeltidae and Phloiophilidae are never carinate.Phloiophilidae is possibly the morphologically most ancestral family among the mentioned extant taxa as indicated by the absence of hooked spur on the protibia and also characteristics in the male genitalia and larval head.Conversely, the tiny body and especially peculiar prothoracic structures indicate that Foveapeltis, although rather primitive in other morphological structures, was highly adapted to some specialized ecological niche and mode of life.
As we are not able to confidently determine the position of Foveapeltis, here we decide to leave it as Cleroidea incertae sedis.The most eye-catching character of Foveapeltis is the presence of a large empty cavity on each hypomeron, which is unique among the whole Cleroidea.A somewhat similar hypomeral cavity is known in the monospecific family Acanthocnemidae (Matsumoto & Geiser, 2021).
However, the cavity in Acanthocnemidae is covered by a sensory disc, serving as an infrared receptor (Kreiss et al., 2005;Schmitz et al., 2002;Zhou et al., 2016).The function of the empty cavity in Foveapeltis is presently unknown.
Extant adults of the trogossitid-like groups are often mycophagous or predatory (Kolibáč & Leschen, 2010).Many of them are F I G U R E 1 0 The most parsimonious placement of Foveapeltis, analyzed based on the full matrix.Tree resulting from the constrained parsimony analysis under implied weights.

(
26°20′ N, 96°36′ E), Hukawng Valley, Kachin State, northern Myanmar.The specimen NIGP203568 is deposited in the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences, Nanjing, China.The remaining specimens (four amber pieces) are deposited in the Institute of Zoology, Guangdong incident light.(a) Dorsal view; (b) Ventral view.Scale bars: 500 μm. the result by Li et al. (2021) is more accordant to the phylogenomic studies by McKenna et al. (2019) and Cai et al. (2022).It should be nevertheless noted that the result by Li et al. (

Etymology.
The species is named after the coleopterist Dr. Rafał Ruta (University of Wrocław, Poland), an expert especially in the systematics of Scirtidae.Locality and horizon.Amber mine located near Noije Bum Village, Tanai Township, Myitkyina District, Kachin State, Myanmar; unnamed horizon, mid-Cretaceous, Upper Albian to Lower Cenomanian.Diagnosis.As for the genus (vide supra).

Foveapeltis
differs from all the abovementioned lineages in terminal tarsomere approximately as long as other tarsomeres together and elytra with distinct longitudinal carinae.The long terminal tarsomere is shared with the remaining cleroid groups including Rentoniidae, Phloiophilidae, Protopeltidae, Trogossitidae, Thymalidae, Peltidae and Lophocateridae, which were all historically classified in a broadly circumscribed Trogossitidae(Kolibáč, 2013).Foveapeltis shares very small body size (1-2 mm) with Rentoniidae, but the latter group always has a conglobate body with smooth dorsal surface(Gimmel & Leschen, 2014;Lawrence & Ślipiński, 2013).
R E 11 Constrained parsimony analyses showing alternative placements of Foveapeltis.The score above each branch represents the parsimony score of the topology in which Foveapeltis is inserted to that branch.(a) Analysis under equal weights.(b) Analysis under implied weights.